“That day was the most terrifying day of my life," she said, choking back tears as she remembered the panic of that afternoon. “You have to prepare yourself because you don't know what you’re going to see.”

The EF5 tornado that swept through Moore on May 20, 2013 killed 24 people -- including seven children -- and injured nearly 400 others.

About two dozen students and parents were hurt at Briarwood, but no one died.

“If you would have saw what I saw,” McCalister said, “there is no way that you could have thought all of those kids came out of there alive.”

Amanda McCalister had three kids at Briarwood Elementary in Moore, Okla. She says the day a tornado flattened the school was "the most terrifying day of her life." No children at Briarwood were killed, but seven children at nearby Plaza Towers school died.NBC News

Since then, teams of researchers have been analyzing the damage to see if structures can be better protected from future tornadoes.

An engineer at the University of Oklahoma, Chris Ramseyer, told the state legislature he spotted possible construction flaws at Briarwood, which had been built about 30 years ago.

Ramseyer said the steel rebar in the school’s cinder block walls was “inadequate” -- only overlapping six to eight inches in some spots when it should have overlapped at least 24 inches.

He also said at least one support beam was not properly connected to the wall.

The findings are now being peer-reviewed by the American Society of Civil Engineers, which has not issued its final report on its investigation. Lynn Wallace, a spokeswoman for the group, would not comment.

Moore school officials have insisted that Briarwood and other schools in the district were built up to code.

"The schools were built years ago and at the time they were built they were built to those building codes," said Jimi Fleming, the spokesman for Moore Public Schools. “We are looking forward to what the peer-reviewed report says.”

A University of Oklahoma engineer, Chris Ramseyer, told the state legislature that the steel rebar in Briarwood’s cinder block walls was “inadequate” -- only overlapping six to eight inches in some spots when it should have overlapped at least 24 inches. He also said at least one support beam was not properly connected to the wall.Chris Ramseyer / Univ. of Oklahoma

The ASCE’s Structural Engineering Institute has investigated several disasters, including the 2011 Joplin, Mo., tornado, the 2011 Japanese tsunami, the 2011 New Zealand earthquake and Hurricane Sandy in 2012.

The seven students who died were inside another school, Plaza Towers Elementary, when the walls of its third-grade center collapsed. The ASCE team did not study its construction because the school was being demolished at the time of the team’s visit to Moore.

Experts note that it’s nearly impossible to protect a structure against a maximum-strength EF5 tornado. But after decades of focusing on hurricane-prone regions, researchers are pushing to strengthen building codes throughout Tornado Alley to withstand low-level twisters.

Andrew Graettinger, an associate professor of engineering at the University of Alabama, was part of another team that conducted a separate study of the Moore tornado funded by the National Science Foundation. Graettinger studied wood structures, not schools.

"People are starting to look at designing for tornadoes,” he said. “I think it's going catch on across the country in tornado-prone areas."

The tornado that swept through Moore killed 25 people -- including seven children -- and injured nearly 400 others. About two dozen students and parents were hurt at Briarwood, but no one died. Chris Ramseyer / Univ. of Oklahoma

Among the suggestions: metal straps to hold a building’s roof to its walls and the walls to its foundation -- a common building practice along the Gulf Coast now to protect from hurricanes.

As Graettinger notes, much of the damage from the most powerful storms is actually done by their weaker winds.

“It turns out when we look at the large storms -- those EF5 storms -- the vast majority of the storms, about 85 percent of the area actually is only hit by EF2 (winds),” Graettinger said. “So it’s not as if those very strong winds of an EF5 get the entire area. It's just in the center.”

He says buildings should be constructed to withstand lower-level storms like EF2s. For stronger tornadoes, Graettinger and other researchers say storm shelters are essential.

The American Society of Civil Engineers has not yet issued its final report on its investigation of construction at Briarwood.Chris Ramseyer / Univ. of Oklahoma

FEMA has been recommending since at least 2002 that schools should consider constructing community shelters or safe rooms in schools. The Department of Commerce’s National Institute of Standards and Technology (NIST) recommended the same last November.

After the tornadoes in 1999, the Kansas Division of Emergency Management, which administers FEMA’s mitigation programs, and the Kansas Hazard Mitigation Case Studies Team determined that the best use of mitigation funds would be to construct tornado shelters in Kansas schools.

Now, after voters approved a bond measure years later, the Wichita Public School District has two safe room projects under construction, which will serve approximately 7,800 of the District’s 9,000 students.

Teachers carry children away from Briarwood Elementary in Moore, Okla., after a a tornado destroyed the school on May 20, 2013.PAUL HELLSTERN / The Oklahoman via AP file

In Oklahoma, Briarwood Elementary and Plaza Towers are both being rebuilt with shelters.

But Amanda McCalister, who’s still rebuilding her destroyed home, hopes her kids never have to use them.

"You are never really going fully understand weather,” she said. “It's unpredictable. But I still think it all has to be investigated and looked into so maybe in can prepare us for the future."